1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and to an electrophotographic image forming method and apparatus using the photoreceptor.
2. Discussion of the Background
Electrophotographic image forming methods such as Carlson process and modified process thereof have been widely used for image forming apparatus such as copiers and printers. Among the photoreceptors used for these image forming apparatus, organic photoreceptors are widely used because of being low-priced and pollution-free, and having good productivity.
Organic photoreceptors are broadly classified into photoconductive resin type photoreceptors including a photoconductive resin such as polyvinylcarbazole (PVK); charge-transfer complex type photoreceptors including, for example, PVK-TNF (2,4,7-trinitrofluorenone); pigment-dispersed type photoreceptors in which a pigment such as phthalocyanine pigments is dispersed in a resin; and functionally-separated photoreceptors including a combination of a charge carrier generation material and a charge carrier transport material. In particular, the functionally-separated photoreceptors attract considerable attention and are widely used.
The mechanism of formation of electrostatic latent images on a typical functionally-separated organic photoreceptor having a photoreceptive layer including a charge carrier generation layer and a charge carrier transport layer are as follows:
(1) when a charged photoreceptor is exposed to imagewise light, the light is absorbed by a charge carrier generation material included in the charge carrier generation layer;
(2) the charge carrier generation material that has absorbed light generates charge carriers;
(3) the charge carriers are injected into a charge carrier transport material included in the charge carrier transport layer; and
(4) the injected charge carriers are transported through the charge carrier transport layer by an electric field caused by the charging of the photoreceptor, and finally neutralize the charge formed on the surface of the photoreceptor, resulting in formation of an electrostatic latent image on the photoreceptor.
Organic photoreceptors preferably have the following properties:
(1) good charge properties, i.e., formability of high electric potential and good charge maintaining ability;
(2) good potential decay properties when exposed to light, i.e., high photosensitivity and low residual potential;
(3) good spectral properties;
(4) good mechanical strength; and
(5) good chemical resistance to heat, light, gases caused by charging such as ozone and NOX.
Recently, small-size image forming apparatus are desired, and therefore the diameter of photoreceptors becomes smaller and smaller. In addition, recently high speed digital image forming apparatus in which laser light is used as a light source of an imagewise light irradiation device have been developed and used. Therefore, the time during which a photoreceptor is exposed to imagewise light also becomes shorter and shorter. Therefore a need exists for a photoreceptor having high photoresponse.
In order to prepare a photoreceptor having high photoresponse, designing of proper formulation of the photoreceptive layer is important as well as development of high sensitive photoconductive materials. However, investigation of the formulation of the photoreceptive layer has not been satisfactorily performed. In addition, it has not been performed to establish the method of evaluating photoresponse of photoreceptors.
Japanese Laid-Open Patent Publications Nos. 8-6450 and 8-62862 have disclosed photoreceptors useful for high speed image recording, which include an organic charge carrier transport material having a charge mobility in a specified range. However, when the factor of thickness of the charge carrier transport layer is not considered, a problem in which resolution of the resultant images deteriorates or the resultant photoreceptor does not have high photoresponse tends to occur.
Further, recently long-life photoreceptors are also desired even when the thickness of the photoreceptors is relatively thin to effectively produce images having good resolution. Therefore, the durability (i.e., resistance to abrasion) of photoreceptors becomes more important. In general, organic materials used for photoreceptive layers have poor resistance to abrasion, and therefore a photoreceptor having good resistance to abrasion has not been obtained yet.
Japanese Laid-Open Patent Publications Nos. 4-287052 and 5-165384 have disclose photoreceptors which include a component capable of decreasing the friction coefficient of the photoreceptors. In addition, Japanese Laid-Open Patent Publications Nos. 6-342236 and 9-81001 have disclose image forming apparatus in which a device applying a component, which decreases the friction coefficient of the photoreceptor thereof, is provided. However, these techniques have not been applied to a photoreceptor used for high speed recording.
Japanese Laid-Open Patent Publication No. 8-272198 discloses an image forming apparatus in which the charge mobility of the photoreceptive layer, the thickness of the photoreceptive layer, the moving speed of the image bearing member configured to face the photoreceptor, and the electric field formed in the photoreceptive layer are set at proper values so as to satisfy a specific relationship to avoid formation of tailing of images. However, the image forming apparatus having such a structure cannot perform high speed recording.
Because of these reasons, a need exists for a photoreceptor which can exhibit an excellent combination of high photoresponse and good durability so as to be used for high speed image forming apparatus.
Accordingly, an object of the present invention is to provide a photoreceptor which can exhibit an excellent combination of high photoresponse and good durability so as to be used for high speed image forming apparatus.
Another object of the present invention is to provide an image forming method and apparatus which can produce good images at a high speed even upon use of a photoreceptor for a long period of time.
To achieve these objects, the present invention contemplates the provision of an electrophotographic photoreceptor including an electroconductive substrate and a photoreceptive layer, which is formed overlying the substrate, wherein the photoreceptive layer includes a charge carrier generation layer including a charge carrier generation material and a charge carrier transport layer including a charge carrier transport material, and wherein when an electric field of from 2.5xc3x97105 V/cm to 5.5xc3x97105 V/cm is applied to the charge carrier transport layer, the following relationship is satisfied:
xe2x80x83tCTL/xcexcCTLxe2x89xa61.5xc3x97103 Vxc2x7s/m
wherein tCTL represents a thickness of the charge carrier transport layer and xcexcCTL represents a charge mobility of the charge carrier transport layer.
The charge carrier transport material preferably includes a high-molecular-weight charge carrier transport material.
In addition, the surface of the photoreceptor preferably has a friction coefficient not greater than 0.5.
Another aspect of the present invention is to provide an electrophotographic image forming method including the steps of charging the photoreceptor mentioned above, irradiating the charge photoreceptor with imagewise light to form an electrostatic latent image, and developing the latent image with a developer to form a visible image.
Yet another aspect of the present invention is to provide an electrophotographic image forming apparatus (process cartridge) including at least the photoreceptor mentioned above, a charging device, an imagewise light irradiating device, a developing device, an image transferring device and an image fixing device. The apparatus preferably has a friction coefficient controlling device which controls a friction coefficient of a surface of the photoreceptor preferably by applying a lubricant to the surface of the photoreceptor
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of embodiments of the present invention in conjunction with the accompanying drawings.